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Coexisting amphiboles in an eclogite from the Western Alps: new constraints on the miscibility gap between sodic and calcic amphiboles |
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| Authors: | B REYNARD M BALLÈVRE |
| Institution: | Laboratoire de Minéralogie Physique et Laboratoire de Tectonique, Centre Armoricain d'Etude Structurale des Socles, Universitéde Rennes 1, 35042 Rennes Cedex, France |
| Abstract: | Abstract Crystal-chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the Aosta Valley, Western Alps. Textural analysis gives evidence of three successive high-pressure parageneses: 1. Pre-kinematic high-grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A). 2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B). 3. Post-kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C). P–T conditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar. Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from Na The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end-members. |
| Keywords: | amphiboles eclogite miscibility gaps Western Alps |